Well I have been in the same boat for a few weeks now, 4g really inconsistent! After speaking to Verizon for a week, they could not figure out whats going on. They finally told me they would send me a replacement yesterday. I use my phone for business and need internet on my phone so I decide to try different things to solve this problem. I finally decided to take out the SD card and try it out. IT WORKS. I tested it last night it worked great, once I put the SD card back in, no 4g??? Today, with SD card, no 4g with no SD card 4g has been working great. I have not tried using a different SD card but I thought I would share!
Try it and let me know, no sd card and 4g should be consistent!